Molded conductive plastic resistor and method of making same



March 8, 1966 D. N. SHAHEEN 3,239,789

MOLDED GONDUOTIVE PLASTIC RESISTOR AND METHOD OF MAKING SAME Original Filed July 2, 1962.

INVENTOR. 3 W fa/wl W 422 United States Patent 3,239,789 MOLDED CONDUCTIVE PLASTIC RESISTOR AND METHOD OF MAKING SAME Daniel N. Shaheen, Boston, Mass, assignor to Ace Electronics Associates, Inc, a corporation of Massachusetts Continuation of application Ser. No. 207,476, July 2, 1962. This application Oct. 10, 1963, Ser. No. 320,249 Claims. (Cl. 338-162) This application is a continuation of application Ser. No. 207,476, filed July 2, 1962, now abandoned.

The present invention relates generally to molded conductive plastic resistors, and particularly to such resistors for use in potentiometers or rheostats.

A number of compositions for and methods of fabricating conductive plastic resistors are taught in the prior .art. These resistors are generally formed by combining a non-conductive synthetic resin with a conductive powder, molding the mixture into the desired shape and size, and curing the same to provide the desired resistance element. It is known to fabricate such resistors with varying specific or volume resistivities by varying the ratio of conductive powder to non-conductive powder, excellent unit or fixed value resistors can be readily fabricated possessing the desirable physical characteristics of the resin, such as strength, resistance to shock and wear, and ability to withstand reasonably high temperatures.

With the development of these unit or fixed value molded plastic resistors, the industry has sought to develop corresponding types of variable resistors, such as potentiometers or rheostats. For many applications it has been found that the mere conversion of suitable unit type resistors to rheostats is entirely unsatisfactory because of the high electrical noise characteristic and electrical irregularities of such devices resulting from the traverse of the resistor by the rheostat wiper contact. It is believed that this high noise characteristic and the irregularities result at least in part from discontinuities between the conductive particles on the surface of the resistor, resulting in relatively large fluctuations in the tapped voltage value as the wiper contact traverses the unit.

The noise problem may be overcome by fabricating units of very low specific resistivity, such as by using a very high percentage of conductive powder relative to the resin. Loading the unit with conductive powder however causes the resulting device to take on the physical characteristics of the conducting material rather than the plastic, and the value of the device as a conductive plastic resistor is in large measure defeated. Other solutions to the noise problem are available, provided the specific resistivity of the unit is kept very loW. By the present invention however, it has been found that conductive plastic resistors may be fabricated having exceedingly low noise characteristics, and that this low noise characteristic can be had with plastic resistors having very high specific resistivities and a relatively low percentage of conductive particles in the resistor composition.

In the preferred forms of the present invention the resistor is molded from a mixture of a phenol aldehyde thermosetting resin as the non-conductive resin, and carbon black as the conductive powder material. However, other types of moldable resins may be employed in place of or in conjunction with the preferred phenol aldehyde resin, and similarly, powders of other conductive materials may be employed in conjunction with or in place of the carbon.

A basic feature of the present invention resides in the manner in which the surface of the resistor wiper 3,239,789 Patented Mar. 8, 1966 contact track, that is the area of the resistor surface that is traversed by the rheostat or potentiometer wiper, is formed. In general, in accordance with the present invention, the wiper contact track surface is formed from a fluid mixture of the resin and conductive powder in a solvent for the resin, the resin being dissolved in the solvent and the power being homogeneously dispersed in the solution. The mixture is deposited as a thin layer in the desired configuration of the rheostat or potentiometer wiper contact track, and the solvent is evaporated under a temperature and over a period of time which effects partial cure of the resin. The residual film is then molded to a base, which may be composed either entirely of non-conductive plastic, or include an additional resistive current carrying portion. The film and base are comolded under curing heat and molding pressure, with a smooth mold surface applied along the wiper contact surface of said deposited layer.

The current carrying wiper tracks of resistors produced in accordance with the present invention are strong, tough and plastic-like in physical characteristics, displaying properties similar to the resin employed. Further, when used as potentiometers or rheostats, they exhibit very low wiper contact noise, and retain their characteristics even after prolonged usage. In addition, these resistors have been produced with a wide range of resistivity characteristics.

It is accordingly one object of the present invention to provide a molded conductive plastic resistance unit, having the physical properties of plastic and a low surface noise characteristic when traversed by a wiper contact.

Another object of the present invention is to provide .a surface layer for molded conductive plastic resistors, having the physical properties of plastic and a low surface noise characteristic when traversed by a wiper con tact. i

Further objects of the present invention are to provide molded conductive plastic resistors which display low wiper contact surface noise characteristics, and hence are particularly adapted to be used as potentiometers and rheostats, and to provide methods of producing these resistors.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following exemplary specific examples thereof had in conjunction with the accompanying drawings, in which like reference characters refer to like or corresponding parts, and wherein:

FIG. 1 is a vertical sectional and partial perspective view of a mold employed in fabricating one form of resistor in accordance with the present invention;

FIG. 2 is a cross-sectional and partial perspective view of a plastic resistor fabricated from the mold of FIG. 1 in accordange with prior art teachings;

FIGS. 3 and 4 are cross-sectional and partial perspective views of two embodiments of plastic resistors fabricated in accordance with the present invention in the mold of FIG. 1; and

FIG. 5 is a molded resistor potentiometer fabricated in accordance with the present invention.

For the purposes of illustration, the form of the resist ance unit described in this specification is chosen to be a raised annular track mounted on a circular base, so as to be adapted for use in a rotary type potentiometer or rheostat. Accordingly a representative type of mold for forming the unit is shown in FIG. 1. The mold comprises an annular sleeve 10 housing a bottom or base member 11, defining a molding cavity 14 thereabove. Base member 11 has a recess 13 therein for defining the resistance track. A plunger 12 is axially slidably received in the cavity 14. When the mold cavity is appropriately charged with a plastic molding composition and the plunger 12 is positioned on top thereof, the charged mold is placed between a pair of heated pressure platens acting to compress the charge through the plunger 12 and heat cure the same, all as fully understood by those skilled in the art.

In accordance with the teachings of the prior art, a plastic resistor may be formed and molded in the mold of FIG. 1 by first charging the annular track 13 in base 11 with a molding thermosetting resin powder, such as phenol aldehyde, wherein the individual powder grains have been clad with a conductive powder, such as carbon black. The base is inserted in the sleeve 10, and the mold is then further charged with a thermosetting resin molding powder, such as phenol aldehyde, devoid of conducting material, to fill a portion of the cavity 14. Plunger 12 is inserted in sleeve 10, and the mold is inserted between the platens of a heated molding press to compress the two resin charges between plunger 12 and base 11, to comold and to heat cure them. A resistance unit is produced as shown in FIG. 2, having a comolded insulating base 15, and a conductive resistive portion 18 along the rib 16.

Resistance units produced in accordance with the foregoing procedure have been found atisfactory for potentiometer use, provided the specific resistivity of the conductive resin portion 18 is very low. On the other hand, resistance units manufactured in this manner having relatively high specific resistivities are quite unsatisfactory for many potentiometer uses because they exhibit a high amount of electrical noise with very high intermittent resistance spikes when traversed by a potentiometer wiper contact.

As previously stated, a basic principle of the present invention is to provide a high resistivity resistance track which provides a low electrical surface noise characteristic for a traversing potentiometer wiper contact. In accordance with the specific embodiment of the present invention, the resistance element or its surface layer is formed in the mold, by depositing in track 13 a fluid solution of a thermosetting resin such as phenol aldehyde and solvent therefor, having conductive particles such as carbon black homogeneously admixed therewith. This deposited layer is then dried of solvent and partially cured by heating at about 300 F. for 20 minutes. The mold base element 11 with the partially cured resin-carbon layer is then inserted in sleeve 10, surcharged with a desired quantity of thermosetting resin molding powder, such as phenol aldehyde, and then the combined charge is comolded under heat and pressure between base 11 and plunger 12 in a suitable press. The resultant resistance unit is shown in FIG. 3, having a conductive resistance layer 17 on the surface of ridge 16, carried by the insulating resin base 15.

The foregoing procedure is particularly suited for the manufacture of resistance elements having a very high specific resistivity, and providing such elements with a surface having a very low electrical noise characteristic when traversed by a potentiometer wiper contact. To fabricate a similar resistance unit of moderately high specific resistivity, the layer 17 is formed in the same manner as described immediately above, and after the partial cure, a small quantity of carbon clad thermosetting resin molding powder, such as phenol aldehyde, i added in track 13 on top of the layer 17. Base 11 is inserted in sleeve 10, and then the combined charge is surcharged with the insulating thermosetting resin molding powder. The three layers are then comolded under heat and pressure between base 11 and plunger 12 in a molding press to produce the resistance unit of FIG. 4. This unit comprises an insulating base 15 formed with ridge 16, having a first conductive layer 18, superposed by a second conductive layer 17. The latter layer 17 provides a low electrical noise contact surface for a potentiometer wiper.

It will be observed from the foregoing description that three conditions apply to the formation of layer 17 in the resistance units of FIGS. 3 and 4. These are: (l) deposition of the layer as a fluid solution of resin in solvent having the conductive particles homogeneously admixed therewith; (2) drying and partial curing of the deposited layer; and (3) the comolding and full curing of said layer, while still in contact with a molding urface, with a resin base, to unite the layer with the base.

The following Examples I and II are specific examples of the procedures for fabricating the resistance units of FIGS. 3 and 4 respectively, in accordance with the present invention.

Example I A homogeneou solution of a phenol aldehyde one stage thermosetting resin (Durez #175 resin) in cyclohexanone is formed, having 40 grams of resin in grams of solvent. To this solution is added 10 grams of a conductive oil furnace carbon black (Vulcan XC72 obtained from Godfrey L. Cabot, Inc.), which has been previously dried by baking for 16 hour at 150 C. and then cooled under vacuum. The resultant mixture is mixed in a blender to form a lumpless homogeneous fluid mixture. A layer of this mixture of carbon black and resin dissolved in solvent is deposited in the recessed track 13 of mold base 11, such as by spraying or extrusion from a nozzle, and the layer is then heated in an oven at 300 F. for 20 minutes to evaporate the solvent out of the deposited layer and partially cure it. The

ried partially cured layer forms a film or deposit along the bottom portions of the recess 13. The base member 11 and sleeve 10 are then assembled as shown in FIG. 1, the mold cavity is charged to a desired depth with a phenol aldehyde molding powder, such as Durez #14684, the plunger 12 is inserted, and the mold is placed in a press, heated to about 390 F. and subjected to a molding pressure on the piston 12 and base 11 of about 230 p.s.i. After 4 minutes the pressure is increased to 6,000 p.s.i. and held at this value for 25 minutes, whereupon the mold is removed from the press, disassembled, and the molded unit removed. To stabilize the unit, it is baked at C. for 1 hour, and then returned to room temperature over a 5 hour period. This baking step is repeated three times.

The resultant resistor unit is shown in FIG. 3, and comprises a non-conductive polymerized phenol aldehyde base 15 having a raised ridge 16. On the surface of the ridge 16 is the conductive layer 17 of polymerized phenol aldehyde containing carbon black. It should be noted that the layer 17 was formed by first being dried and partially cured with what was to become its wiper contact surface in contact with the mold recess 13, this recess of course having been machined smooth; and when pressure molded and fully cured, the contact surface was likewise in pressure contact with the surface of recess 13. Thus, the resistor is a layer which is originally formed for a fluid solution of the resin in a solvent, the solution containing a desired proportion of conducting powder, and the resistor was finished by comolding under full curing conditions with the base member, the surface of the resistor being formed by pressure contact with the corresponding mold surface.

The resistor produced by Example I is a strong plasticlike solid with good toughness, and physically is very similar to the polymerized phenol aldehyde when molded alone. When employed as a potentiometer with a conventional metal wiper contact, the wiper surface contact noise is very low, for example 30 ohms and free from intermittent resistance spikes, in a 58K resistor with .9% linearity. The molded resistance ridge was about three inches long.

A similar resistance unit of lesser specific resistivity can be prepared by combining two procedures described above.

Example I] A dried partially cured resistance layer 17 is formed in the groove 13 of base member 11 as described in Example I. On top of layer 17 groove 13 is filled with a quantity of carbon clad phenol aldehyde molding powder formulated by ball milling 85 grams of phenol aldehyde molding powder (such as I-Ieresite M6610) with 15 grams of a conductive oil furnace carbon black (e.g. Valcan XC-72 obtained from Godfrey L. Cabot, Inc.), which has been previously dried by baking for 16 hours at 150 C. and then cooled under vacuum. The base member 11 and sleeve are assembled as shown in FIG. 1, the mold cavity is charged to a desired depth over the layer 17 and the superposed carbon clad resin layer with a phenol aldehyde molding powder (such as Durez #14684), the plunger 12 is inserted, and the charge is fully comolded and cured pursuant to the molding and stabilizing cycles set forth in Example I.

The resultant resistor unit is shown in FIG. 4, and comprises a non-conductive polymerized phenol aldehyde base having a raised ridge 16. In the upper portion of ridge 16 is a first conductive stratum 18 of polymerized phenol aldehyde containing carbon black, as formed from the carbon clad resin powder; and on the surface of the ridge, overlying the stratum 18, is the conductive layer 17 of polymerize-d phenol aldehyde containing carbon black. As in the preceding example, the layer 17 was formed by depositing a fluid resin, solvent, carbon mixture and drying and partially curing the deposit with what became its potentiometer wiper contact surface in contact with the mold recess 13; and the comolding and full curing of the final unit was likewise had with this wiper contact surface in contact with the recess 13.

The resistor produced by Example II is a strong plastic-like solid with good toughness, and physically is very similar to the polymerized phenol aldehyde when molded alone. When employed as a potentiometer with a conventional metal wiper contact, the Wiper surface contact noise is very low, for example 30 ohms and free intermittent resistance spikes, in a 9.5K resistor, with 1% linearity. The molded resistance ridge was about three inches long as in the preceding example, the same mold being used in each instance.

By way of comparison, a resistance unit was fabricated in the same mold by molding the same carbon clad resin molding powder as prepared for Example II, i.e. that used to form the layer 18, with the same molding cycle as used for Example II. The resulting unit, which was thus equivalent to the unit of Example II except for the surface layer 17, had an overall resistance of 12K for three inches of length, with an average noise level of about 40-50 ohms and intermittent spikes of 100 to 150 ohms, and a linearity of about 4%.

It has been found that analogous results are obtained over a Wide range of resistivities, and that regardless of the electical noise and discontinuities or irregularities that may be obtained when using the layer 18 alone, the provision of a contact surface 17 thereover, in accordance with the procedures of the present invention, results in a low noise potentiometer having good linearity, and enables the production of such potentiometers having a wide range of high resistivities.

It is not intended to restrict the scope of the present invention to the specific examples herein particularly described, for other modifications of the processes and re sistors described will be apparent to those skilled in the art. For example, other thermosetting non-conducting molding resins than phenol aldehyde can obviously be employed; similarly, other types of carbon black, and powdered conducting materials other than carbon can be employed; all as is well understood by those skilled in the art. The proportions of conductive powder to resin can be varied over wide ranges from those given in the specific examples, since the choice of proportions is utilized to obtain the resistivity characteristics desired. It is considered, however, that the particular procedure of forming the wiper contact surface of layer 17 is critical in establishig the low noise contact surface characteristics of the resistors of the present invention.

In large measure, the purpose for providing resistors with surfaces having low wiper contact noise characteris tics, is for use in potentiometers and rheostats. Accordingly, FIG. 5 shows a resistance unit of the present invention embodied in a potentiometer. The resistance unit comprises the integrally molded insulating or non-conductive base 15 with the current carrying resistance 17 thereon. The ends of the resistor 17 are terminated in appropriate terminals 21 and 22. An aperture in base 15 on the center of the resistor arc mounts a shaft 20 carrying Wiper 19 having the contact element 23. Rotation of shaft 20 causes contact element 23 to traverse the low noise surface 17 of the resistor, and the contact 23 thus affords a variable tap for the potential applied acres the resistor through terminals 21 and 27.

Having thus described the present invention both generally and specifically, and in such terms and examples as would enable one skilled in the art to practice the same, it is intended that the scope of the invention shall be determined from the spirit and intent of the appended claims.

What is claimed is:

1. A method of forming a molded resistor unit, comprising the steps of: depositing a layer of a solution of a thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith, in a shape approximating that desired for the resistor; baking the deposited layer at curing temperature for the resin to evaporate the solvent and effect a partial cure of the resin; and comolding in a mold and fully curing said deposited layer with a base material therefor comprised of a thermosetting resin; said comolding step being effected with a surface of said layer in pressure contact with a smooth molding surface of the mold.

2. A method as set forth in claim 1, wherein said powdered conductive material is carbon.

3. A method as set forth in claim 1, wherein said moldable resin for said layer and for said base member is a phenol aldehyde resin.

4. A method as set forth in claim 1, wherein said powdered conductive material is carbon, and wherein said moldable resin for said layer and for said base member is a phenol aldehyde resin.

5. A method as set forth in claim 1, wherein said baking step is effected at a temperature of about 300 F.

6. A method of forming a molded resist-or unit, comprising the steps of: depositing on a mold surface a layer, of a solution of a thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith, in a shape approximating that desired for the resistor; baking the deposited layer at curing temperature for the resin to evaporate the solvent and effect a partial cure of the resin; charging the mold with a thermosetting resin material adapted to provide a base member for said layer; and comolding and fully curing said deposited layer with said base material.

7. A method as set forth in claim 6, wherein said base material is charged into said mold as a molding resin powder.

8. A method as set forth in claim 6, wherein said baking step is effected at a temperature of about 300 F.

9. A method as set forth in claim '6 wherein said layer is overlayed with a thermosetting resin powder clad with conducting particles after said baking step and before said charging step.

10. A molded resistance unit comprising: a base formed of thermosetting non-conducting resin material; and a molded current carrying resistor carried by said base; at least the surface of said resistor having been formed from a deposited layer of a solution of a thermosetting resin in a solvent having admixed therewith a powdered conductive material; said layer having been first baked at a curing temperature for the resin to evaporate the solvent and partially cure the resin, and then comolded in a mold and fully cured with said base, with the surface of said layer in molding contact with a smooth surface of said mold.

11. A molded resistance unit as set forth in claim 10, wherein said resistor includes in addition to said deposited layer a further current carrying section, whereby the current carried by said resistor is divided between said layer and section.

12. A potentiometer comprising: a molded resistance unit having a base formed of thermosetting non-conducting resin material; and a molded current carrying resistor carried by said base; at least the surface of said resistor having been formed from a deposited layer of a solution of a thermosetting resin in a solvent having admixed therewith a powdered conductive material; said layer having been first baked at a curing temperature for the resin to evaporate the solvent and partially cure the resin, and then comolded in a mold and cured with said base, with the surface of said resistor in molding contact with a smooth surface of said mold; and a wiper arm having a wiper contact member mounted to traverse the resistor; the surface of said resistor having a low surface noise characteristic when traversed by said wiper contact.

13. A potentiometer as set forth in claim 12, and further including a pair of terminals in electrical contact with spaced points of said resistor for applying a potential across said resistor.

14. A potentiometer as set forth in claim 12, wherein said resistor includes in addition to said deposited layer a further current carrying section, whereby the current carried by said resistor is divided between said layer and section.

15. A method of forming a molded resistor, comprising the steps of: depositing a layer of a solution of a thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith; baking the deposited layer at a curing temperature for the resin to evaporate the solvent and effect a partial cure of the resin; and thereafter molding in a mold and fully curing said deposited layer with a surface of said layer in pressure contact with a smooth molding surface of the mold.

16. A method as set forth in claim 15, wherein said powdered conductive material is carbon, and said resin is a phenol aldehyde resin.

17. A method as set forth in claim 16, wherein said baking step is effected at a temperature of about 300 F.

18. A molded resistor produced by the process set forth in claim 15.

19. A molded resistor produced by the process set forth in claim 16.

20. A molded resistor produced by the process set forth in claim 17.

References Cited by the Examiner UNITED STATES PATENTS Re. 18,818 5/1933 Rubin 338217 1,889,379 11/1932 Rubin 338-217 1,992,529 2/1935 Henry 3382l7 2,056,928 10/1935 Magdz'iarz 338-217 2,134,870 11/1938 Fruith 338-2l7 2,700,719 l/l955 Coler et al. 338--308 X RICHARD M. WOOD, Primary Examiner. 

1. A METHOD OF FORMING A MOLDED RESISTOR UNIT, COMPRISING THE STEPS OF: DEPOSITING A LAYER OF A SOLUTION OF A THERMOSETTING RESIN IN A SOLVENT HAVING A POWDERED CONDUCTIVE MATERIAL HOMOGENEOUSLY ADMIXED THEREWITH, IN A SHAPE APPROXIMATING THAT DESIRED FOR THE RESISTOR; BAKING THE DEPOSITED LAYER AT CURING TEMPERATURE FOR THE RESIN TO EVAPORATE THE SOLVENT AND EFFECT A PARTIAL CURE OF THE RESIN; AND COMOLDING IN A MOLD AND FULLY CURING SAID DEPOSITED LAYER WITH A BASE MATERIAL THEREFOR COMPRISED OF A THERMOSETTING RESIN; SAID COMOLDING STEP BEING EFFECTED WITH A SURFACE OF SAID LAYER IN PRESSURE CONTACT WITH A SMOOTH MOLDING SURFACE OF THE MOLD. 